| Quantum information processing has attracted a lot of attention in recent years because of the promise it holds for faster, better, and more secure future communications. The most advanced field in quantum information processing is quantum cryptography, also referred to as quantum key distribution (QKD), which uses the quantum properties of light to ensure the unconditionally secure transmission of a secret message between two parties. Despite the significant progress achieved in the performance of quantum cryptography systems, the communication distance has been limited to a few tens of kilometers and the communication speed remains very low, preventing the integration of these systems into current telecommunication networks. The main limiting factors are the vulnerability of existing QKD algorithms to powerful eavesdropping attacks, and the characteristics of the single-photon detectors employed in the system.; This work addresses both of these limiting factors. We introduce and prove the security of a new quantum cryptography algorithm, the differential phase shift QKD protocol, which requires a very simple system architecture and only standard telecommunication components, such as lasers, detectors, and linear optics. The security proof against the most general attacks allowed by quantum mechanics reveals that this protocol is very robust to powerful eavesdropping attacks. Furthermore, we develop a new single-photon detector, which combines frequency up-conversion in a periodically poled lithium niobate waveguide and a silicon avalanche photodiode to achieve high speed and efficient single-photon detection in the telecommunication wavelength band. By combining these key elements of a quantum cryptography system, we demonstrate the experimental realization of practical and efficient fiber-optic QKD systems, with which we achieved communication at a rate of 2 Mbit/s over 10 km, and transmission of secure messages over 100 km of optical fiber. Compared to existing systems, these results represent an improvement of more than two orders of magnitude in communication speed and a factor of two in communication distance. Thus, they demonstrate that high speed and long distance secure quantum communication is possible with currently available technology, and open the way for real-world applications of quantum information processing. |
